Hybrid refrigeration by CO2 vapour compression cycle and water-based adsorption chiller: An efficient combination of natural working fluids

Abstract

Sustainable refrigeration systems are of great importance to reduce greenhouse gas emissions. In particular, CO2 vapour compression cycles are very promising due to their environmentally friendly, natural refrigerant. However, a major challenge for implementing CO2 cycles is the low efficiency at high ambient temperatures resulting from high exergy losses in transcritical operation. To increase the efficiency, we present a hybrid system concept integrating an adsorption chiller into the CO2 cycle. The adsorption chiller employs the natural refrigerant water and is driven by waste heat from the CO2 cycle. The additional cooling generated by the adsorption chiller is integrated into the CO2 cycle to increase the efficiency of the overall hybrid system. Compared to a stand-alone CO2 cycle, we show by dynamic modelling and optimisation that the hybrid system leads to annual energy savings of 22% for a warm climate in Athens and of 16% for a moderate climate in Cologne. The results highlight the high potential of the hybrid system concept to efficiently provide refrigeration using environmentally friendly refrigerants.